1. Field of the Invention
The present invention relates an image forming system in which a plurality of image forming apparatuses is cascaded serially, and a method for controlling image forming operations to be performed in the image forming system in which the plurality of image forming apparatuses is cascaded serially.
2. Description of Related Art
A tandem-method image forming system is configured by serially cascading a plurality of image forming apparatuses. Hereinafter in this specification, a term of “tandem method” is simply defined as such a connection mode in which a plurality of image forming apparatuses is coupled to each other in a serial connection mode. In the tandem-method image forming system above-mentioned, it is possible for one of the plurality of image forming apparatuses to continuously output a recording paper sheet, on a partial area of which an image is already formed thereby, to a next one of the plurality of image forming apparatuses. Then, the next one of the plurality of image forming apparatuses, included in the tandem-method image forming system concerned, may form another image onto another partial area of the recording paper sheet inputted therein.
According to the image forming system above-mentioned, it becomes possible to implement a high-speed printing operation by employing two image forming apparatuses for respectively printing images onto obverse and reverse sides of the recording paper sheet. Further, it also becomes possible to output the recording paper sheet, having high definition images, in a high-speed printing mode by employing two image forming apparatuses for respectively printing different color images onto the recording paper sheet.
Further, in a tandem-method image forming system constituted by two image forming apparatuses, by making one of the two image forming apparatuses form character images in a character image area, while making another one of the two image forming apparatuses form a pictorial image in a pictorial image area, it is possible to output the recording paper sheet, having high definition images, in a high-speed printing mode. In other words, by employing two image forming apparatuses for constituting the tandem-method image forming system, it becomes possible to improve the productivity thereof.
On the other hand, in an image forming apparatus employing the electro-photographic method, a toner image is formed on a recording paper sheet through consecutive processes including a charging process, an exposing process, a developing process, a transferring process, a fixing process and so on. Further, in order to maintain such the processes at appropriate conditions, a target value of toner adhesion amount corresponding to the maximum density level (hereinafter, referred to as a maximum-density target toner adhesion amount) is established for implementing the maximum density adjustment. Concretely speaking, according to the instructions issued by the control section, the charging voltage (electric potential of the photoreceptor member) and the developing bias voltage are adjusted so as to achieve the maximum-density target toner adhesion amount concerned.
In the above-mentioned case, even though the same value of the maximum-density target toner adhesion amount is established for each of the image forming apparatuses included in the tandem-method image forming system, and the same values of the charging voltage and the developing voltage are also established for each of the image forming apparatuses included in the tandem-method image forming system, the same maximum density level is not necessary achieved among the image forming apparatuses concerned, due to the individual differences therebetween.
In this connection, Japanese Patent Application Laid-Open Publication 2007-137012 sets forth a technology for adjusting the toner adhesion amount to be adhered onto the recording paper sheet by the image forming apparatus.
Japanese Patent Application Laid-Open Publication 2007-137012, above-cited, proposes a method for outputting a plurality of inspection-use charts printed onto the same side of the same paper sheet by changing conditions. In this connection, according to Japanese Patent Application Laid-Open Publication 2007-137012, above-cited, the plurality of inspection-use charts is printed over a single side of the paper sheet while changing gamma characteristics to be employed for image processing, without changing the process conditions.
However, with respect to the maximum density adjustment, it is necessary to halt the operation of the image forming apparatus and implement the adjusting operation by changing the process conditions, such as electric potentials of various sections, etc., instead of gamma characteristics to be used for the image processing. In other words, it is impossible to change the process conditions, such as a charging voltage, a developing voltage, etc., in midcourse of implementing a printing operation onto a single paper sheet. Owing to the above-mentioned fact, it is necessary to output an inspection-use chart onto another paper sheet every time when changing an adjusting value.
Accordingly, in the tandem-method image forming system, it is necessary to individually implement the maximum-density adjusting operation for every one of the image forming apparatuses included therein, and then, it is necessary to compare the inspection-use charts, outputted by the image forming apparatuses concerned, with each other, so as to make the maximum density levels coincide with each other. It has been apparently cumbersome and ineffective to implement the above-mentioned adjusting operations. However, no method for effectively implementing the maximum-density adjusting operation in the tandem-method image forming system as above-defined, has been proposed so far.
Further, in the tandem-method image forming system as above-mentioned, with respect to a density level other than the maximum density level, for instance, a certain intermediate density level (hereinafter, also referred to as a halftone density level), a cumbersome adjusting operation being same as the maximum density adjusting operation above-mentioned may become necessary. Namely, even for the halftone density level, no method for effectively implementing a halftone density adjusting operation in the tandem-method image forming system as above-defined, has been proposed so far.